The reference implementation, CPython, is released under the Python Software Foundation License, an OSI approved licence. There are bindings for most popular platform-independent GUI toolkits, including:
As for an IDE, since you mentioned Eclipse, PyDev is probably what you're looking for. I prefer ActiveState's Komodo IDE, but it's not free.
The core language and the standard library are ANSI/ISO standards, and although not strictly an open source language, all major C++ compilers are free to use:
There are quite a few open source bindings / interfaces for most popular platform-independent GUI toolkits, just pick one and search for it. As for an IDE, well, Eclipse with CDT would be my choice.
Mono is multi-licensed under the MIT licence, GPLv2 and LGPLv2. When it comes to Microsoft's patents, insofar they've followed their promise, but who knows what the future will bring. Mono allows you to work with a variety of open source GUI toolkits, and MonoDevelop seems to be the IDE of choice.
FreePascal is a Pascal / Object Pascal compiler, licensed under the GPL. The Lazarus Component Library (LCL) and fpGUI are the more popular FreePascal GUI toolkits, and Lazarus the IDE of choice.